The present invention relates to a colour switch for selectively switching between different colour bands for use, for example, in projection optics and in direct view optics.
Colour switches based on retarders and liquid crystal (LC switches) are known and are shown for example in U.S. Pat. No. 5,528,393. They are however difficult to produce. To get the desired steep spectral transmission curves requires a large number of retarders, which are expensive to produce. Another approach is to use retarders in high order (thick retarders with a retardation xcex94nxc2x7dxe2x89xa72xcex, where d is the thickness, xcex94n the birefringence and xcex the wavelength of light). Since production variations of retarder thicknesses are usually proportional to the thickness, very tight tolerances are necessary to meet the required accuracy (which is typically xc2x150 nm).
Colour switches based on cholesteric filters and LC switches (stacked band modulation filters, BMFs) are shown for example in U.S. Pat. No. 4,726,663. These are less sensitive to manufacturing tolerances. The performance of these devices depends strongly on the birefringence An of the liquid crystal polymer (LCP) material used to produce the cholesteric filters in the BMFs. The most elegant approach is the incorporation of high-xcex94n materials (xcex94n greater than 0.3) into the filters, since then only 6 cholesteric layers (two for each colour) are required. However, cholesteric filters made from large xcex94n-materials exhibit strong side-lobes in their transmission spectra, reducing the overall transmission of the filter stack. Furthermore, high-xcex94n materials tend to absorb more light, especially in the blue region of the spectrum, limiting the lifetime in high-light-intensity applications such as projection systems. If low-xcex94n materials are used considerably more layers are required (e.g. for xcex94n=0.12, six times more layers are necessary: three times more to cover the spectral range; since low-xcex94n materials also require much thicker layers, which cannot be produced in a single step, a further doubling of the layers results). A further drawback of stacked BMFs is their subtractive colour generation scheme: they divide the total spectrum in non-overlapping sub-spectra of the three primary colour R, G, B. While this leads to high colour purity, it is not the most light-efficient approach, because an overlap between 490 nm and 500 nm between the green and the blue spectra yields excellent colour saturation with significantly higher light throughput than non-overlapping configurations.
The concept of the invention is to employ a cholesteric filter (more generally chiral filter) to selectively block one of the primary colours, and optimised retarder combinations are used to block the remaining primary colours. Such an arrangement provides a colour switch with fewer and thinner layers than in the prior art, but which exhibits equal or better colour purity and light efficiency. Thus, the invention combines the good properties of cholesteric filters (sharp spectra) with the good properties of birefringent filters (higher brightness than purely subtractive filters). Fewer layers implies fewer manufacturing steps; thinner layers implies less tight manufacturing tolerances.
The present invention provides in a first aspect a colour filter for switching between three different colour bands characterised by the combination of a first filter section for selectively blocking a first colour band and a second filter section for selectively blocking second and third colour bands;
the first filter section including a chiral filter means for reflecting one circularly polarised state of the first colour band and transmitting the other circularly polarised state;
and the second filter section including a combination of a plurality of retarder elements and a plurality of electrically controlled liquid crystal switches, arranged so as to be switchable between a first state in which both the second and third colour bands are blocked, a second state in which the second colour band is blocked, and a third state in which the third colour band is blocked.
The filter comprises a stack of elements, and preferably the chiral filter comprising the first filter section is placed at the front of the stack for the incoming light beam. Whilst it may be placed at the back of the stack, this may cause a reduced switching contrast because light reflected from the chiral filter may be re-reflected with reversed polarisation by the filter sections in front of the cholesteric filter.
The second filter section might in principle be constructed as two sub-sections, each sub-section acting on a respective primary colour band. However, in accordance with the invention it is preferred to use as switches in the second section, deformed helical ferroelectric switches (DHF). These have various variable parameters such as voltage dependent birefringence and dispersion, cell gap, orientation of optical axis. In addition, optical retarders have various parameters which may be varied, for example birefringence and dispersion, thickness and orientation. It is preferred in accordance with the invention rather than designing the second filter section separately for respective primary colour bands, to optimise the properties of the various elements of the second filter section bearing in mind its various parameters. This confers significant advantages in terms of overall thinness of the colour switch and excellent filter properties (sharp spectra, high brightness).
Thus in a further aspect, the invention provides, for a colour filter for selectively switching first, second and third colour bands, and including a first filter section for selectively blocking a first colour band, and a second filter section for selectively blocking at least a second colour band, the first and second sections including a combination of a plurality of retarder elements and electro-optic elements, a method of optimising the characteristics of the colour filter comprising
a. defining parameters of the elements of the first filter section to define the first colour band;
b. determining the variable parameters of the elements of the second filter section for optimisation;
c. minimising with respect to the parameters of the second filter section a cost function G:   G  =                    ∑                              F            =            R                    ,          G          ,          B                    ⁢                        g          IF                ⁢                  (                                                    (                                                      x                    F                                    -                                      x                    F0                                                  )                            2                        +                                          (                                                      y                    F                                    -                                      y                    F0                                                  )                            2                                )                      +                  (                              g                          2              ⁢              F                                ⁢                      (                                          Y                F                            -                              Y                F0                                      )                          )            2      
where YF is brightness, and xF, yF are the colour co-ordinates, xF0, yF0 and YF0 are respectively target values for colour co-ordinates, brightness and wherein the parameters g are weight coefficients, including adjusting recursively the weights g of such that after optimisation the terms in G are of generally similar magnitude.
In regard to the first filter section, a quarter wave plate is used to convert the circularly polarised light of the primary colour component to linearly polarised light, and a liquid crystal switch in combination with a polarising sheet is employed to selectively switch the primary colour component. The liquid crystal switch either rotates the optical axis (such as a DHF cell) or switches between birefringent and non-birefringent state (such as a Pi-cell).
It will often be advantageous to choose for the cholesteric filter a selective reflection band that lies inside the overall colour spectrum. For an RGB system for instance green, which lies between red and blue, may be chosen since the sharp edge of the cholesteric filter spectrum on both sides can be used.
A colour switch according to the invention can be used in projection optics and in direct view optics. The display may consist either of a single pixel or a multitude of pixels, or of a combination of a xe2x80x9csingle pixelxe2x80x9d colour switch with a pixellated (grey-scale) display.
A problem arising in projection systems is that they must tolerate extremely high light intensities with no degradation. In this regard, the first filter section, including a polarising sheet is a potential problem since it absorbs radiation. It is therefore preferred to use a non-absorbing reflective polariser. Polarisation recovery schemes are known using non-absorbing polarisers that split unpolarised light into two beams of differently polarised light, and then transform the polarisation of one beam into the polarisation of the other and combine them to a single beamxe2x80x94see for example U.S. Pat. No. 5,235,443. However, such polarisers are relatively bulky and it is therefore preferable to position the polariser outside of the stack formed by the colour switch according to the invention.
In accordance with a further aspect of the invention, there is provided a chiral filter section in which the elements of the filter section are so organised that it is possible to place a polarising element of the filter section in a leading position to receive an incoming beam of light, with the remaining elements of the filter section following the polarising element and arranged in a stack.
Advantageously, the liquid crystal elements of the colour filter, that is chiral filter, retarders and switches, are aligned by a photo-orientation technique. Among the different known methods particularly well suited will be those using linear photopolymerisation (LLP), also sometimes referred to as photooriented polymer network (PPN). Backgrounds and manufacturing of such elements are disclosed in, for example, U.S. Pat. Nos. 5,389,698, 5,838,407, 5,602,661, EP-A-689084, EP-A-0756193, WO-A-99/49360, WO-A-99/64924, and WO-A-00/36463.